In order to provide cores for electromagnetic structures which are subjected to alternating current fields, it has been proposed to make the cores of stacks of electrical sheet-metal elements, in which the respective sheet-metal elements are insulated from each other in order to reduce eddy losses. To provide for precise alignment of stacks which also include winding slots, so that the windings can readily be placed into the stacks, it has been proposed to deform the respective sheet-metal elements or lamellae by punch marks, in the form of blind holes, which, at the opposite sides of the sheet-metal elements, will result in essentially cylindrical bumps or projections. It has been found that, upon interengaging the projections of one sheet-metal element with the blind hole of the next adjacent one, electrical connection between the superimposed sheets may result, thus increasing eddy current losses of the overall package.